1. Field of the Invention
This invention relates to heat curable organosiloxane compositions. More particularly, this invention relates to one-part organosiloxane compositions exhibiting both excellent storage stability in the vicinity of room temperature and excellent curing characteristics at elevated temperatures.
2. Background Information
Organosiloxane compositions which cure by a hydrosilation reaction between at least one alkenyl-containing organopolysiloxane and at lest one SiH-containing organohydrogenpolysiloxane are distinguished by the absence of reaction by-products and by rapid curing in deep sections. These advantages have led to their use in a wide range of applications such as adhesives, potting materials for electrical and electronic components, and release coatings for paper and films.
However, this type of organosiloxane composition suffers from a very poor storage stability and cannot be stored in a single container. Rather, the composition must be stored in two parts, with the organohydrogenpolysiloxane and the platinum-containing catalyst in separate containers.
Prior art methods for overcoming this disadvantage are based on restraining or inhibiting the catalytic activity of the catalyst. One such method uses additives that inhibit the activity of platinum-containing catalysts. Inhibitors of this type include benzotriazole, acetylenic compounds, and hydroperoxy compounds. When long-term storage stability is sought using these inhibitors, the curing characteristics of the composition are degraded and the time required for curing is lengthened. While a very storage-stable organopolysiloxane composition can be obtained, a fast curing organopolysiloxane composition cannot be obtained.
A second approach to one-part organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction involves the use of a platinum-containing catalyst that is microencapsulated or otherwise enclosed within at least one layer of a thermoplastic organic polymer or a thermoplastic silicone resin. The melting or softening temperature of the encapsulating material is selected to correspond with the temperature at which the curable composition is heated during the curing process.
One method for encapsulating platinum-containing catalysts is described in Japanese Patent Publication [Kokai] Number 49-134786 [134,786/74], which teaches preparing a platinum-type catalyst in powder form. Silicone resin and a platinum compound adsorbed on a finely divided material, such as quartz powder, are mixed and kneaded on a hot roll, and the resulting mixture is pulverized or ground.
U.S. Pat. No. 4,481,341, which issued to Schlak et al. on Nov. 6, 1984 describes thermosetting organosiloxane compositions comprising a polyorganosiloxane containing at least two ethylenically unsaturated hydrocarbon radicals per molecule, a polyorganohydrogensiloxane containing at least two silicon bonded hydrogen atoms per molecule and a platinum-containing catalyst that is present in a finely divided, solid material, such as a silicone resin. The concentration of catalyst is equivalent to from 0.001 to 5 percent by weight of platinum metal, based on the weight of the curable composition. In accordance with the examples of this patent, the catalyst is dispersed in a solution of a silicone resin. The solution is then cast in the form of a sheet and the solid evaporated to form a solid that is subsequently pulverized to obtain a powder.
The finely divided material in which the catalyst is dispersed is virtually insoluble in either the aforementioned polyorganosiloxane or polyorganohydrogensiloxane and melts or softens at a temperature between 70 and 250 degrees C. The alleged advantage of the compositions disclosed by Schlak et al. is that the catalyst remains isolated from the other ingredients of the curable composition until the composition is heated sufficiently to melt or soften the material in which the catalyst is dispersed. Because the organosilicon composition will not cure in the absence of the catalyst, the composition can allegedly be stored for long periods of time without undergoing curing or even an increase in viscosity.
U.S. Pat. No. 4,784,879, which issued to Lee et al. on Nov. 15, 1988 describes the preparation of an encapsulated form of a platinum-containing hydrosilation catalyst. The platinum-containing catalyst is encapsulated within one or two layers of thermoplastic organic polymers. The catalyst is prepared by polymerization or precipitation of the encapsulating polymer in the presence of the catalyst. The resultant microcapsules are then washed with a liquid that is a solvent for the catalyst but not for the encapsulating polymer. The exemplified compositions require one washing with methanol and one with cyclic dimethylsiloxane oligomers to ensure their storage stability when incorporated into a curable organosiloxane composition.
The characteristic feature of all the materials described in the prior art for encapsulating platinum-containing hydrosilation catalysts is the ability of the materials to melt or soften within a specified temperature range.
Particulate forms of cured silicone rubber exhibiting diameters in the range of from 0.1 to about 500 micrometers that incorporate mineral oil or other liquid materials as releasable additives are known. The cured silicone rubber portion of these particulate materials are not considered thermoplastic materials that would be expected to undergo softening or melting in the range of from 100.degree. to 250.degree. C. typically employed for the curing of organosiloxane compositions by a platinum-catalyzed hydrosilation reaction.
Cured silicone rubber is typically not characterized by an abrupt change in its rheological properties in the range of from 100.degree. to 250.degree. C. that would allow release of a liquid material entrapped within the rubber. One would therefore not expect cured silicone rubber to be a useful encapsulating material for platinum-containing hydrosilation catalysts in one-part organosiloxane compositions of the types disclosed in the aforementioned Schlak and Lee et. al. patents.
An objective of the present invention is to provide a thermosetting organopolysiloxane composition which has an excellent storage stability in the vicinity of room temperature yet cures rapidly upon heating.